The invention relates to enthalpy wheels and systems containing enthalpy wheels.
A fuel cell can convert chemical energy to electrical energy by promoting a chemical reaction between two gases.
One type of fuel cell includes a cathode flow field plate, an anode flow field plate, a membrane electrode assembly disposed between the cathode flow field plate and the anode flow field plate, and two gas diffusion layers disposed between the cathode flow field plate and the anode flow field plate. A fuel cell can also include one or more coolant flow field plates disposed adjacent the exterior of the anode flow field plate and/or the exterior of the cathode flow field plate.
Each flow field plate has an inlet region, an outlet region and open-faced channels connecting the inlet region to the outlet region and providing a way for distributing the gases to the membrane electrode assembly.
The membrane electrode assembly usually includes a solid electrolyte (e.g., a proton exchange membrane, commonly abbreviated as a PEM) between a first catalyst and a second catalyst. One gas diffusion layer is between the first catalyst and the anode flow field plate, and the other gas diffusion layer is between the second catalyst and the cathode flow field plate.
During operation of the fuel cell, one of the gases (the anode gas) enters the anode flow field plate at the inlet region of the anode flow field plate and flows through the channels of the anode flow field plate toward the outlet region of the anode flow field plate. The other gas (the cathode gas) enters the cathode flow field plate at the inlet region of the cathode flow field plate and flows through the channels of the cathode flow field plate toward the cathode flow field plate outlet region.
As the anode gas flows through the channels of the anode flow field plate, the anode gas passes through the anode gas diffusion layer and interacts with the anode catalyst. Similarly, as the cathode gas flows through the channels of the cathode flow field plate, the cathode gas passes through the cathode gas diffusion layer and interacts with the cathode catalyst.
The anode catalyst interacts with the anode gas to catalyze the conversion of the anode gas to reaction intermediates. The reaction intermediates include ions and electrons. The cathode catalyst interacts with the cathode gas and the reaction intermediates to catalyze the conversion of the cathode gas to the chemical product of the fuel cell reaction.
The chemical product of the fuel cell reaction flows through a gas diffusion layer to the channels of a flow field plate (e.g., the cathode flow field plate). The chemical product then flows along the channels of the flow field plate toward the outlet region of the flow field plate.
The electrolyte provides a barrier to the flow of the electrons and gases from one side of the membrane electrode assembly to the other side of the membrane electrode assembly. However, the electrolyte allows ionic reaction intermediates to flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly.
Therefore, the ionic reaction intermediates can flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly without exiting the fuel cell. In contrast, the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly by electrically connecting an external load between the anode flow field plate and the cathode flow field plate. The external load allows the electrons to flow from the anode side of the membrane electrode assembly, through the anode flow field plate, through the load and to the cathode flow field plate.
Electrons are formed at the anode side of the membrane electrode assembly, indicating that the anode gas undergoes oxidation during the fuel cell reaction. Electrons are consumed at the cathode side of the membrane electrode assembly, indicating that the cathode gas undergoes reduction during the fuel cell reaction.
For example, when hydrogen and oxygen are the gases used in a fuel cell, the hydrogen flows through the anode flow field plate and undergoes oxidation. The oxygen flows through the cathode flow field plate and undergoes reduction. The specific reactions that occur in the fuel cell are represented in equations 1-3.
H2xe2x86x922H++2exe2x88x92xe2x80x83xe2x80x83(1)
O2+2H++2exe2x88x92xe2x86x92H2Oxe2x80x83xe2x80x83(2)
H2+O2xe2x86x92H2Oxe2x80x83xe2x80x83(3)
As shown in equation 1, the hydrogen forms protons (H+) and electrons. The protons flow through the electrolyte to the cathode side of the membrane electrode assembly, and the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly through the external load. As shown in equation 2, the electrons and protons react with the oxygen to form water. Equation 3 shows the overall fuel cell reaction.
In addition to forming chemical products, the fuel cell reaction produces heat. One or more coolant flow field plates are typically used to conduct the heat away from the fuel cell and prevent it from overheating.
Each coolant flow field plate has an inlet region, an outlet region and channels that provide fluid communication between the coolant flow field plate inlet region and the coolant flow field plate outlet region. A coolant (e.g., liquid de-ionized water or other low conductivity fluids) at a relatively low temperature enters the coolant flow field plate at the inlet region, flows through the channels of the coolant flow field plate toward the outlet region of the coolant flow field plate, and exits the coolant flow field plate at the outlet region of the coolant flow field plate. As the coolant flows through the channels of the coolant flow field plate, the coolant absorbs heat formed in the fuel cell. When the coolant exits the coolant flow field plate, the heat absorbed by the coolant is removed from the fuel cell.
To increase the electrical energy available, a plurality of fuel cells can be arranged in series to form a fuel cell stack. In a fuel cell stack, one side of a flow field plate functions as the anode flow field plate for one fuel cell while the opposite side of the flow field plate functions as the cathode flow field plate in another fuel cell. This arrangement may be referred to as a bipolar plate. The stack may also include monopolar plates such as, for example, an anode coolant flow field plate having one side that serves as an anode flow field plate and another side that serves as a coolant flow field plate. As an example, the open-faced coolant channels of an anode coolant flow field plate and a cathode coolant flow field plate may be mated to form collective coolant channels to cool the adjacent flow field plates forming fuel cells.
In some fuel cell systems, an enthalpy wheel is used to transfer energy and/or water from one gas stream to another, such as from an anode gas outlet stream to a cathode gas inlet stream. This can improve the efficiency of the fuel cell system by reducing the amount of water and/or energy that is added to the system during operation.
The invention relates to enthalpy wheels and systems containing enthalpy wheels.
In certain embodiments, the enthalpy wheels are designed so that a relatively constant pressure is applied to the rotor within the enthalpy wheel as the temperature of the enthalpy wheel varies from room temperature to the operating temperature of the enthalpy wheel. This can reduce the variation in the torque required to rotate the wheel during operation.
In some embodiments, the enthalpy wheels are designed so that a relatively low torque is used to rotate the enthalpy wheel during use. This can reduce the amount of energy that is input into a fuel cell system containing the enthalpy wheel during use of the fuel cell system, thereby increasing the overall efficiency of the fuel cell system.
In one aspect the invention features an enthalpy wheel subassembly that includes a first plate, a cap, a compliance element between the first plate and the cap and a pressure producing element between the first plate and the cap. The pressure producing element being different from the compliance element. In some embodiments, the enthalpy wheel subassembly is included in an enthalpy wheel, which, in turn, can be included in a fuel cell system, such as a PEM fuel cell system.
In another aspect, the invention features an enthalpy wheel subassembly that includes an enthalpy wheel subassembly housing having a first end and a second end, a plate at the first end of the enthalpy wheel subassembly, a cap at the second end of the enthalpy wheel subassembly, a compliance element between the first plate and the cap and a pressure producing element between the first plate and the cap, the pressure producing element being different from the compliance element. In some embodiments, the enthalpy wheel subassembly is included in an enthalpy wheel, which, in turn, can be included in a fuel cell system, such as a PEM fuel cell system.
The enthalpy wheel subassembly can include one or more of the following features.
The compliance element can be partially or completely formed of foam.
The pressure producing element can be one or more springs.
The enthalpy wheel subassembly can further include a second plate between the first plate and the cap. The compliance element can be between the first and second plates. The pressure producing element can be between the second plate and the cap.
In certain embodiments, the invention features methods of operating one or more of the preceding enthalpy wheels or a fuel cell system (e.g., a fuel cell system containing PEM fuel cells) to generate water and power (e.g., electricity).
Other features, aspects and advantages of the invention will be apparent from the description, the drawings and the claims.